In the production of semiconductors, during the manufacturing process, wafers are sequentially processed in a plurality of process steps. As integration densities increase, the requirements as to the quality of the structures formed on the wafer become ever more demanding. To be able to verify the quality of the structures formed and to find any defects, the requirements as to the quality, the precision and the reproducibility of the components and process steps for handling the wafer are correspondingly stringent. This means that in the production of a wafer including a great number of process steps and with a great number of layers of photoresist to be applied, the reliability and early detection of defects is particularly important. In the optical detection of defects, it is a question of taking into account systematic defects due to thickness variations in the application of photoresist on the semiconductor wafer, so as to avoid marking positions on the semiconductor wafer that do not include a defect.
By means of a so called macro inspection, semiconductor surfaces are optically scanned to detect defects. In this way, for example, wafer surfaces are scanned. The detected defects are visually shown on a wafer overview image. In some of the continuous processing steps the resulting overview images are very weak in contrast, so that the defects can often only be detected with difficulty. However, algorithms are known with the help of which defects can be found and identified as such, which cannot be reliably detected with the human eye and identified as a defect. Since automatically detected defects, or defects detected by means of suitable algorithms must be checked by a so-called operator and confirmed as a defect, if any, it is desirable to improve the imaging quality of the detected defects.
German Patent Application DE 10 307 454 A1 discloses a method, an apparatus and a software for the optical inspection of the surface of a semiconductor substrate, and a method and an apparatus for manufacturing a structured semiconductor substrate with the use of such a method or such an apparatus. In the method, for optical inspection an image is recorded of the surface of a semiconductor substrate. The image consists of a plurality of image points (pixels), each having at least three associated intensities of different wavelengths, which are referred to as color values. From the color values, by means of a transformation into a color space, which is defined by an intensity and by color coordinate values, a frequency distribution of image points having the same color coordinates is calculated. The thus calculated frequency distribution is used for a comparison with a second correspondingly calculated frequency distribution or a quantity derived therefrom. This method does not enable a visual comparison or a visual evaluation of a disk-like substrate.
Macroscopic images of semiconductor wafers show that the homogeneity of the layers changes radially. In particular in the application of photoresist there are varying homogeneities in the areas remote from the center point of the wafer. If a uniform sensitivity is used across the entire radius of the wafer, as has hitherto been the case for evaluating images of the imaged wafers, deviations near the edge may always be detected, while defects in the interior (close to the center point of the wafer) are not detected. If a high sensitivity is selected in order to reliably detect defects in homogeneous areas, erroneous detections increase in the edge regions, since the inhomogeneous edge regions are not always to be evaluated as defects. In order to avoid this, the edge regions can be completely omitted. This means, however, that no real defects are found there. If a reduced sensitivity is chosen, however, there are no more erroneous detections, but defects in the homogeneous areas cannot be found.
German Patent Application DE 103 31 686.8 A1 discloses a method for evaluating recorded images of wafers or other disk-like objects. The recording of the image of at least one reference wafer is followed by obtaining and visualizing the radial distribution of the measuring values of the reference wafer as a radial homogeneity function on a user interface. A sensitivity profile as a function of the radius is changed while taking the measured radial homogeneity function of the reference wafer into account. At least one parameter of the sensitivity profile is varied, allowing a trained sensitivity profile to be visually determined from the comparison with the radial homogeneity function. This method, again, does not show an image of the entire wafer with the aid of which the image or the images are then evaluated with respect to defects.
U.S. Pat. No. 7,065,460 discloses an apparatus and a method for inspecting semiconductor components. The apparatus is for inspecting the electrical characteristics of the semiconductor product. The measuring values obtained from the inspection are associated with various colors for visualization on a display.
The illustrative display of measuring values in the form of graphs in diagrams only makes sense for one dimension of the distribution of measuring points. If the measuring points are distributed in space, they must be reduced to one dimension in a visual image. This involves a loss of information. A visualization in a 3-D plot does not always lead to an illustrative image, since some points may be covered by others. It is very difficult to establish a link between the initial information and the measuring values. The representation in the form of numbers does not allow conclusions to be drawn as to the spatial distribution of the measuring values.
To improve the imaging quality of the optically detected wafer surfaces, image processing methods have been known for some time with the aid of which the resulting images can be processed to facilitate subsequent evaluation and to make the process of decision finding by an operator more reliable and less prone to error. However, subsequent image processing methods slow down the processing and inspection speed since image recording and image processing are separated on a temporal and sometimes on a local basis. For this reason a rapid evaluation of the recorded images is often not possible. If, for example, wafer surfaces have been found which make a further processing of the wafer in question appear at least partially doubtful, it may happen that the production process must be interrupted to wait for the subsequent evaluation of the images. The establishment and optimization of recipes is thus made difficult.